3-3.Randomization.A randomized clinical trial was designed to compare two different treatment approaches for irritable-bowel syndrome. The purpose of randomization in this study was to:

1. increase patient compliance with treatment.
2. obtain comparison groups that are similar on other variables that may influence the disease.
3. obtain comparison groups that are similar on any other variables measured in the study.
4. Increase the likelihood of finding a significant effect of treatment.
5. obtain a representative sample in the study.

3-9

3-9.Randomization: Clinical Trials.What is the purpose of randomization in a clinical trial? [Choose one best answer.]

a.To make the diseased and non-diseased as similar as possible with respect to all variables except the exposure of interest

b.To reduce the number of subjects who are lost to follow-up

c.To isolate the effect of the exposure of interest

d.To encourage compliance with the assigned treatment regimen

8-3

.Selective Survival

1. What is meant by "bias due to selective survival" in cross-sectional studies? (In your answer, make sure to define appropriate selection probability parameters.)
2. Under what circumstances might there be no selective survival bias even if the selection probabilities are not all equal?
3. Suppose that you could assess that the direction of possible selective survival bias in your study was towards the null. If your study data yielded a non-statistically significant odds ratio of 1.04, would it be correct to conclude that there was no exposure-disease association in your source population? Explain.

8-6

8-6.Identifying Study Subjects

A study entitled "Antidepressant Medication and Breast Cancer Risk" (Amer. J. of Epi, late 1990's) stated in the methods section of the paper that "Cases were an age-stratified (< 50 and 50 years of age) random sample of women aged 25-74 years diagnosed with primary breast cancer during 1995 and 1996 (pathology report confirmed) and recorded in the population-based Ontario Cancer Registry. As the 1-year survival for breast cancer is 90%, surrogate respondents were not used. Population controls, aged 25-74 years, were randomly sampled from the property assessment rolls of the Ontario Ministry of Finance; this database includes all home owners and tenants and lists age, sex, and address."

1. Discuss the authors' approach to the identification of cases with respect to the potential for selection bias.
2. Discuss the authors' approach to the identification of controls with respect to the potential for selection bias.
3. Does the decision not to allow surrogate respondents potentially affect selection bias?

9-3

9-3.Sleep Disturbance vs. Clinical Depression

The Johns Hopkins Precursors Study, a long-term prospective cohort study, was used to evaluate the relation between self-reported sleep disturbances and subsequent clinical depression.A total of 1,053 men at several U.S. universities provided information on sleep habits during medical school and then were followed for 20 years for development of depression.Subjects underwent extensive psychological testing and lengthy structured interviews conducted by psychiatrists trained in the diagnosis of depression.Results of the study are summarized below (you may assume that these data are correctly classified)

Sleep Disturbance

Sleep Disturbance

Yes No

Depression 168 93

No Depression 258 534

426 627

Now suppose that the investigators had limited funds and were not able to use the sophisticated diagnostic tools described above.They had two options available to them for diagnosing depression among the study subjects: (1) an interview or (2) a self-administered questionnaire.

The interview was able to classify depression status with the following sensitivities and specificities:

Exposed:Sensitivity = 0.84 and Specificity = 0.90

Unexposed:Sensitivity = 0.84 and Specificity = 0.90

The questionnaire was able to classify depression status with the following sensitivities and specificities:

Exposed:Sensitivity=0.80 and Specificity = 0.88

Unexposed:Sensitivity = 0.90 and Specificity = 0.95

1. What is the true relative risk (RR) that describes the association between sleep disturbance and depression?
2. What is the observed RR when the interview is used to classify depression?
3. What is the observed RR when the questionnaire is used to classify depression?
4. Taking only issues of validity into account, which tool for diagnosing depression is preferable - the interview or the questionnaire?Justify your answer.

9-8

Predictive Value: Diabetes

In a certain community, eight percent of all adults over age 50 have diabetes. If a health service in this community correctly diagnosis 95% of all persons with diabetes as having the disease and incorrectly diagnoses ten percent of all persons without diabetes as having the disease, find the probabilities that:

1. The health service will diagnose an adult over age 50 as having diabetes.
2. A person over 50 diagnosed by the health service as having diabetes actually has the disease.

10-4

Error type

For each of the situations described below, indicate the type of error that would most likely occur. Each of the numbered options can be used once, more than once, or not at all.

i.Selection bias

ii.Nondifferential misclassification

iii.Differential misclassification

iv.Confounding

v.Ecologic fallacy

vi.Random error

a.In a cohort study of hormone replacement therapy (HRT) and risk of atherosclerotic coronary artery disease (CAD), high income level is associated with both HRT use and risk of CAD.

b.In a case-control study of the relation between stressful daily events and asthma attacks, cases are more likely than controls to over-report the amount of stress.

c.In a cohort study of use of video display terminals (VDTs) and risk of carpal tunnel syndrome, the users of VDTs are more difficult to trace than nonusers, resulting in a greater loss to follow-up of VDT users.

d.In a case-control study of beta carotene and risk of esophageal cancer, serum specimens frozen and stored 20 years earlier are compared between cases and controls. Later it is found that the specimens deteriorated while in storage.

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